Run examples/*.py in CI, fix the broken example, and give silent ones output

[voidstackloop]

Which issue does this PR close?

Closes #1728.

Rationale for this change

Nothing in CI runs the top-level examples/*.py scripts, so they silently drift
from the API and from each other. Two consequences are already in the tree:

• examples/csv-read-options.py crashes: it reads data.csv and
  data.csv.gz, neither of which exists in the repository.
• Nine examples end in assert with no output, so a reader cannot tell a
  working example from a script that does nothing.

Adding a CI job is what keeps this from recurring.

What changes are included in this PR?

• examples/csv-read-options.py now writes its own small CSV and gzipped CSV
  into a temporary directory, so it is self-contained and runs without external
  data.
• The nine silent examples (export.py, import.py, python-udaf.py,
  python-udf.py, query-pyarrow-data.py, sql-to-pandas.py,
  sql-using-python-udaf.py, sql-using-python-udf.py, substrait.py) now
  print their result, keeping their existing assertions.
• .github/workflows/test.yml gains a Run example scripts step (Python 3.12
  entry) that runs every examples/*.py against the built wheel. It installs
  the extra runtime dependencies some examples use (pandas, polars,
  matplotlib) and skips an explicit list of examples that cannot run in CI:
  • sql-parquet-s3.py — needs network access and AWS credentials
  • sql-parquet.py, dataframe-parquet.py, sql-to-pandas.py — need the NYC
    taxi parquet file documented in examples/README.md
  • python-udf-comparisons.py — needs the TPC-H dataset generated later in the
    workflow
  • ray_pickle_expr.py — needs the optional (heavy) ray dependency

Are there any user-facing changes?

No API changes. The examples are documentation; they are now runnable, print
their results, and CI will catch it if they break again.

Verification

• Ran every non-skipped example against a datafusion==54.0.0 wheel in a fresh
  venv: all pass. multiprocessing_pickle_expr.py and ray_pickle_expr.py were
  not run locally (Windows has no forkserver and Ray is unavailable there);
  they run on the Linux CI.
• ruff check / ruff format --check (pinned 0.15.1), codespell and
  actionlint (v1.7.12) pass on the changed files.